Envelope delay correction link

ABSTRACT

A circuit for correcting nonuniform transmission delay of a television signal has two parallel coupled channels coupled to receive said signal. The first contains a damped resonant circuit, the second a phase inverter. A MOSFET can be the variable damping, while voltage variable diodes can be in the tuned circuit. A rectifier supplies a control voltage from the input signal to the diodes.

Uited States Patent Peter Lison Lunden;

Anders Gustaf Lyden, both of Jakobsberg, Sweden July 1, 1969 Dec. 14,1971 U.S. Philips Corporation New York, N.Y.

July 2, 1968 Sweden July 10, 1968, Sweden, No. 9525/68 lnventors Appl.No. Filed Patented Assignee Priorities ENVELOPE DELAY CORRECTION LINK 14Claims, 24 Drawing Figs.

U.S. Cl 328/155, 307/295, 328/133, 330/124, 333/76, 333/80 Int. Cl 1103b3/04 Field of Search 328/155, 133; 333/76, 80 T; 330/124, 126; 178/695DC; 307/295 Primary ExaminerDonald D. Forrer Assistant Examiner-R. C.Woodbridge Att0rney- Frank R. Trifari ABSTRACT: A circuit for correctingnonuniform transmission delay ofa television signal has two parallelcoupled channels coupled to receive said signal, The first contains adamped resonant circuit, the second a phase inverter, A

MOSFET can be the variable damping, while voltage variable diodes can bein the tuned circuit. A rectifier supplies a control voltage from theinput signal to the diodes.

irii Patented Dec. 14, 1971 3,628,162

7 Sheets-Sheet 2 INVENTORS PETER L. LUNDEN ANDERS G. LYDEN r F i/ AGEN TPatented Dec. 14, 1971 3,628,162

7 Sheets-Sheet 5 IN VENTORS. PETER L. LUNDEN ANDERS G. LYDEN AGENTPatented Dec. 14, 1971 7 Sheets-Sheet 4.

- INVENTORS. PETER L. LUNDEN ANDERS G. LYDEN AGENT Patented Dec 14, 19713,628,162

7 Sheets-Sheet 5 u fF B (a) t Uv W L Us SV LUL (b) MS r Uin U D A 2 2 -uINVENTORS.

PETER- L. LUNDEN ANDERS LYDEN Patented Dec. 14, 1971 3,628,162

7 Sheets-Sheet 6 INVHNTORS PETER L. LUNDEN ANDERS G. LYDEN AGENTPatented Dec. 14, 1971 3,628,162

7 Sheets-Sheet '7 K7 2 1 Um Uult INVENTORJ PETER L.L.UWDEN ANDERS 6-LYDEN BY AGENT ENVELOPE DELAY CORRECTION LINIK The invention relates toa delay correction circuit having a transfer function the absolute valueof which is substantially independent of the frequency and the phaseangle is dependent on the frequency, comprising a first signal sourcecoupled to a signal input of the delay correction circuit, a secondsignal source coupled to the signal input, a reactance coupled to anoutput of the first signal source and a combination circuit of an outputcoupled to the said signal sources and the reactance from which outputmay be derived the desired output signal whose phase can be influenced.

Such a circuit is known from Gennan Patent specification 1,200,869 andis usually called a delay correction circuit or a phase correctioncircuit, and is adapted to obtain an output signal from an input signalapplied to the circuit which output signal has a constant amplituderatio relative to the input signal, but a frequency-dependent phaseshift. Such a circuit may be used in a signal-handling unit, forexample, in the intermediate frequency section of a TV transmitter. Thecorrection circuit provides a given correction of the delay time withina certain frequency range, but it has the same amplification for allfrequencies. Such a circuit is sometimes called a all-pass circuit. Thecircuit may, for example, consist of only passive (reactive orresistive) elements and is then called a passive correction circuit, orit may consist of both passive and active elements and is then called anactive correction circuit. The correction circuit is required to havethe same amplification for all frequencies within the actual frequencyrange and it must be possible to adjust the delay correction to beobtained in a simple manner both as regards its magnitude and itsposition in the frequency hand without influencing the amplitude ratiobetween the output signal and the input signal.

The invention provides a simple correction circuit which can easily beadjusted and satisfies the requirements for the amplitude with extremelyhigh accuracy for all adjustments of the circuit, and which shows a moresuitable phase charac teristic than the circuits known from the Patentspecification referred to above.

According to the invention a delay correction circuit of the kinddescribed in the preamble is characterized in that the reactanceincludes a resonant circuit.

According to a preferred embodiment of the invention the resonantcircuit includes at least one tuning element which is voltageorcurrent-dependent.

When using a controllable all-pass circuit in which two separatelyproduced signals are combined in a special combination circuit, a phasecorrection produced by the circuit dependent on the amplitude of thefirst signal is possible by which the correction can be adapted tosubstantially any phase error characteristic of an uncorrectedtransmission circuit. Particularly the controllable all-pass circuitaccording to the invention provides a greater correction range than theknown devices for correcting differential phase errors and allows inprinciple phase corrections of the magnitude of i1 80.

The difierential phase error usually occurs on a final stage to which amodulated signal is applied. It is therefore physically correct to alsocarry out the correction on such a modulated signal. The controllableall-pass circuit is therefore preferably incorporated in a section ofthe transmission circuit where the combined signal is modulated on acarrier, for example, on an lF-carrier.

It is advantageous to carry out the phase correction such that therelationship between the supplied control voltage and the produced phasecorrection is linear, the correction being adapted to a nonlinear phaseerror characteristic (phase error as a function of the instantaneousvalue of the first signal) by producing a control voltage which isnonlinearly related to the instantaneous value of the first signal. Fora correction which is carried out on a carrier frequency, for example,intermediate frequency, this can be achieved by generating the controlvoltage by means of a device comprising a number of demodulators whichhave different threshold values each supplying a voltage which afterexceeding the threshold value increases linearly with the detected firstsignal, and an adding device for adding the voltages supplied by thedemodulators, the threshold value of the demodulators and the addingconstants for the different voltages being so adapted that the controlvoltage, which is derived from the output of the adding device,substantially forms the inverse function of the actual phase errorcharacteristic.

The dependency of the delay correction of the first signal produced bythe correction circuit or circuits may be varied by a variation of thetuning of the resonant circuit. This may be effected by influencingcontrollable reactive elements, such as controllable capacitors and/orinductors included in the resonant circuit, dependent on the controlvoltage. It is alternatively possible to vary the Q-value by influencingcontrollable resistive elements such as, for example, field-effecttransistors included in the resonant circuit by means of the controlvoltage.

In order that the invention may be readily carried into effeet, a fewembodiments thereof will now be described in detail by way of examplewith reference to the accompanying diagrammatic drawings, in which:

FIG. 1 diagrammatically shows an allpass circuit according to theinvention,

FIGS. 2(a) and 2(b) illustrate by way of phaser diagrams in what mannerthe circuit according to FIG. I operates,

FIGS. 3(a), 3(b) and 3(0) show amplitude and phase characteristic curvesfor the pass channel circuit of FIG. 1,

FIG. 4 diagrammatically shows a preferred embodiment of the circuitaccording to the invention,

FIG. 5 shows a diagram for a correction circuit according to theinvention as shown in FIG. 4

FIGS. 6 and 7 show different possible modifications of a correctioncircuit according to the invention,

FIGS. 8(a) and 8(b) Show in what :manner a phase or amplitude error inthe correction circuit according to the invention results in certaindisturbances in the amplitude of the output voltage,

FIGS. 9 to 12 show in what manner the errors illustrated in FIG. 8 maybe eliminated by different modifications of the correction circuitaccording to FIG. 5,

FIGS. 13(a) and 13(b) show in what manner the differential phase errorof a chrominance subcarrier may vary with the magnitude of a videosignal on which the subcarrier is su' perimposed,

FIG. 14 shows the shape of the delay curve for different levels of thevideo signal and the delay curve for a correction circuit,

FIG. 15 diagrammatically shows in what manner the cor rection accordingto FIG. 14 may be performed with the aid of a delay correction circuit,

FIG. 16 shows a suitable embodiment of a delay correction circuitaccording to the invention,

FIG. 17 shows a phase diagram for the circuit according to FIG. 16,

FIG. 18 shows a device for correcting the differential phase error whichdevice comprises two delay correction circuits, and the delay curves ofthe device, and

FIG. 19 shows in what manner a desired control voltage can be produced.

The transfer function for an all-pass circuit will generally have theform as shown in equation (I) wherein U the input signal of the circuitand U, the output signal and E is a variable representing the frequency.It is to be noted that the absolute value of the ratio rn/ 1 isconsequently equal to l for all 45 while the phase=shift varies with of2, and on the other hand to a second voltage amplifier A, having a gainfactor of -l. The output voltage of the first amplifier A is appliedthrough a resistor R to a parallel circuit arrangement consisting of acapacitor C and an inductor L. The output voltage U of the said circuitis applied to a first input 1 of a voltage adding device A, The outputvoltage of the second amplifier A, is applied to the second input 2 ofthe voltage adding device A,. The output signal U derived from theoutput of the voltage adding device A and is formed by the sum of thevoltages which are applied to the inpum l and 2.

The following relationship exists for the voltages U at the input 1:

. l l+ R (wc which can be written as wherein:

2 E f o Rq L Q VL/C VLC The sum of the voltages at the inputs 1 and 2will be equal to From this it is evident that equation (III) isidentical to equation (1) with the given definition for .5.

FIG. 2(a) shows equation (ll) represented in the complex plane for somedifferent values of 5. According to FIG. 2 the end of the phasor Udescribes a circle at a variation of from to FIG. 2(b) shows the way ofbroken lines the phasor U-U,,, -U,,, for the same value of 5. As isshown U-U;, is a phasor which starts from the center of the said circleand thus has always the same size, but assumes difierent phase angles.

FIG. 3 shows the amplitude A, the phase d and the delay 1 for thevoltage U as a function of the angular frequency w. According to thedefinition 1" is equal to d/dw.

It can be shown that I T'miiX T 1 +5 wherein 'max=? (lV) wherein Thecurve for the delay of the voltage U as a function of the angularfrequency or has in principle the same shape as imum value of thevoltage U which is applied to the input of FIG. 1 will be equal to U Z UWhen U=U which occurs at resonance, the voltage at input 1 is in phasewith the input voltage U A voltage which according to the foregoingU;,,='U ,/2 is applied to the second input 2 of the adding device A,. Ageneral rule for the correction circuit described is that the outputsignal of a parallel resonant circuit fed with a first input signal becombined with a signal of a constant amplitude which is equal to halfthat of the said first input signal and is in phase oppositiontherewith, the sum of the signals then having an all-passcharacteristic.

The block diagram-of FIG. 1 shows a circuit which has practicaldrawbacks, inter alia, the voltage adding device should in principlehave an infinite resistance and the amplifiers A A, should have anoutput impedance of zero. FIG. 4 shows a preferred embodiment which hasappreciable advantages.

According to FIG. 4 the input voltage U is applied on the one hand to afirst voltage-current amplifier (current generator) A having a gainfactor of 2/R and on the other hand to a second voltage-currentamplifier A, having a gain factor of l/R. The current supplied by thefirst amplifier A. is applied to a parallel resonant circuit consistingof a capacitor C, an inductor L and a resistor R which resistor R isconnected to an input of a current-voltage amplifier A having a gainfactor of --R. The current supplied by the voltage-current amplifier Ais directly applied to the input of the amplifier A The current-voltageamplifier A consists of a voltage amplifier having a high gain factor,the output of which is fed back through a resistor to the input, theinput of adding point P being maintained at a low potential as regardsthe signal voltages. ln the following, point P is assumed to beconnected to earth as regards signal voltages so that the resistor R forsignal voltages is connected in parallel with the capacitor C and theinductor L and forms part of the parallel resonant circuit. The currentI, which flows through the resistor R of the parallel resonant circuitis combined at the input of the amplifier A, with the current I, whichis obtained from the current generator A and the output signal is formedby the sum of these currents multiplied by the constant R.

The circuit according to FIG. 4 has the same transfer function as thecircuit in F l0. 1, which will be shown hereinafter.

in the same manner as for the circuit of FIG. 1, the resistance R isonly present in the imaginary portion of the transfer function and hencewill not influence the maximum value of the signal obtained from theresonant circuit. The maximum value of the delay caused by thecorrection circuit of FIG. 1 or FIG. 4 can be adjusted in a simplemanner by varying a single element (R and the position of this maximumvalue along the frequency scale can likewise be adjusted in a simplemanner by varying a single element, for example, the indicator L or thecapacitor C Furthermore the two variations do not substantiallyinfluence each other. Thus a variation of L does not produce any changeof the maximum value of the delay. it is true that a variation of Ccauses a change of the delay, but in this case, when the correctioncircuits are to be used in intermediate frequency amplifiers for TVsignals and where the relative frequency variation within the band, forexample, between 35 and 40 MHz. is comparatively small, this variationof the maximum value is negligible.

F116. shows in what manner the circuit which is diagrammatically shownin MG. 4i can be obtained. The circuit consists of a first transistor Twhich corresponds to the voltage current amplifier A of H6. at and asecond transistor T which corresponds to the inverting voltage-currentamplifier A: of HG. d. The bwe of the first transistor T is connected toearth while the emitter is connected through a resistor R/Z to an inputterminal to which the input voltage U, is applied. The tuned circuit C,L, R is arranged in the collector circuit of the transistor T theresistor R being connected, as shown in FiG. ll, to the input of theadding amplifier F which is fed baclt through the resistor R. The baseof the second transistor T is directly connected to the input terminalwhile the emitter is connected to earth through a resistor R and adecoupling capacitor C The collector of T is directly connected to thecommon adding point P at the input of the fed back amplifier F. Thereferences R R and R are resistors by which the operating points of thetwo transistors are adjusted, and the references (3,, C and C arecoupling capacitors which have a negligible impedance.

The operation of the circuit is as follows:

The transistor t provides a current i, through the resonant circuitwhich current is equal to wherein R is the emitter resistance of thetransistor T At resonance, the entire current i, flows through R to theadding point P. Acurrent l flows through the transistor T which currentis also applied to the adding point P and is determined by the relationwherein r is equal to the emitter resistance of the transistor T Thecurrents applied through R and through the transistor T are combined inthe point P and the sum of the currents is led through the feedbackresistor R. An output voltage U, is obtained from the amplifier whichoutput voltage is equal to the product of the sum of the currents andthe feedback resistor. In the condition of resonance the output voltagewill be According to the foregoing the amplitude of the output volt ageis constant for all frequencies while the phase and the group delay varyaccording to FIG. 3.

Fig 6 shows a simplified embodiment of the correction circuit accordingto the invention. The circuit according to FIG. 6 comprises onetransistor T which serves as a phase inverter stage and as an impedancetransformer or current generator. The base of the transistor T isconnected to the input terminal to which the input voltage U is appliedwhile the emitter is connected through a resistor R/Z to the negativeterminal. The resonant circuit is arranged in the collector circuit ofthe transistor in the same manner as described hereinbefore. Thetransistor T then provides a current i, through the resonant circuitwhich current is determined by the relation The input terminal isfurthermore directly connected to the adding point 1? through a resistorit. A current i is applied through this branch to the adding point Pwhich current is determined by the relation,

At resonance, the current i, entirely flows through 1R to the addingpoint P. The output voltage: U will then be H6. 7 shows an improvedembodiment of the circuit according to FIG. s. The circuit according toFIG. 7 comprises two transistors T and T which are arranged in cascode.The base of the first transistor T. is directly connected to the inputterminal to which the input voltage U is applied while the emitter isconnected on the one hand through a resistor R to the negative terminalof the voltage source and on the other hand through the same resistor Rto the adding point F. The effective emitter resistance of thetransistor T will then be equal to R/2. A current I" will flow throughthe transistor T, which current is determined by the relation whichcurrent is applied through the transistor T to the resonant circuit. Thesame voltage as the voltage which is applied to the input terminal, thatis to say, U will appear at the emitter of the transistor T.,. Thisvoltage causes a current I which flows through the resistor R to theadding-point P and which is equal to The same output voltage U, as thatin the foregoing is obtained from the output of the adding amplifier.

The circuit according to FIG. 5 and the circuit according to FIG. 7 havethe advantage that the voltage-dependent collector-base capacitances Cand C of the transistors T, and T which feed the resonant circuit areeffectively incorporated therein and therefore have a negligibleinfluence.

When the delay characteristic for a transmission cable must be correcteda large number of the correction circuits described is arranged incascode. The circuits are then separately adjusted as regards size andposition in the frequency band of the additional delay until theresultant delay for the transmission cable and the correction circuitsis constant through the actual frequency range.

If the conditions as to phase and amplitude given in FIG. 2 are notfulfilled, this will give rise to undesired variations in the amplitudeof the output voltage. This is illustrated in FIG. h where the topportion of FIG. 8a shows the case of a small phase deviation Alb betweenthe currents or voltages which are added, that is to say, when thecurrent or voltage which is combined with the output magnitude of theresonant circuit is not exactly in phase opposition with the current orvoltage of the resonant circuit at resonance. This gives rise to anamplitude characteristic which is shown in the lower portion of FIG.8(a). FIG. ti(b) illustrates the case where an amplitude error do ispresent, that is to say, the current or the voltage which is combinedwith the current or the voltage from the resonant circuit is not exactlyequal to half the output magnitude of the resonant circuit at resonance.This gives rise to an amplitude characteristic which is shown in thelower portion ofFlG. 8(1)).

Some causes of the errors illustrated in FIG. h when using a correctioncircuit according to FM. 5 and steps to eliminate these errors will nowbe described with reference to F lGS. 9 to 112;.

In case of inversion in transistor T the base-collector capacitance C ofthe transistor T (FIG. 9), if not negligible, will influence the phaseof the inverted current and thus cause an erroneous mutual phaserelation between the two currents which are combined. According to FIG.9 this can be compensated for by means of a variable resistor r which isconnected to the input line of the transistor T The compensation of theerror is based on the fact that the case has always also a certaincapacitance C relative to earth. It can be proved that for each value ofC and C,,, a value for the resistor r can be found which produces aphase shift such that the current flowing through the transistor T, isexactly in phase opposition with the input voltage. The phase errorwhich is caused by the base-collector capacitance C is independent of Qand the compensation will therefore be correct for all values of Q.

Particularly when using a controllable impedance element such as afield-effect transistor as a resistor R in the oscillator circuit, butalso when using conventional resistors, the resistor R will have anonnegligible parallel capacitance which gives rise to a mutual phaseerror between the currents which are combined. This is illustrated inFIG. 10 where the parallel capacitance of the resistor R is indicated byC According to FIG. 10 this phase error is compensated for in that thecapacitance C of the resistor R is compensated for by center-earthedsymmetric bridge circuit. This circuit is formed because the inductor ofthe resonant circuit is divided into two subinductors L and the junctionof which is connected to earth with respect to signal voltages, and inaddition a variable capacitor C is connected between the end of theresonant circuit which is not connected to the resistor R and the addingpoint P at the input of the amplifier F. The variable capacitance C isadjusted until it is equal to the capacitance C of the resistor of theresonant circuit so that this will apply a current to the adding pointwhich is equal to the current which flows through the parallelcapacitance of the resistor R but is of opposite sign with respectthereto. The phase error which is produced by C is dependent on R thatis to say, the Q of the resonant circuit, but the compensation currentwhich added through the capacitor C varies to the same extent with thisQ and the compensation applies to all Q- values.

FIG. also shows in what manner the capacitance C of the resonant circuitcan be formed by two controllable variable capacity diodes C C, by whichthe resonant frequency of the circuit can be adjusted electronically bymeans of a control voltage applied to the variable capacity-diodes.

An amplitude error of the kind described in FIG. 8(b) may depend on thefact whether the ratio between the resistors R and R/Z is not exactlyequal to the given ratio, or on the fact whether the current gainfactors of the two transistors T, and T are not equal. Such an amplitudeerror is independent of Q and is compensated according to FIG. 11because the emitter resistance of the transistor T has been renderedadjustable. As shown, for example, in FIG. 11, the emitter resistancemay be divided into a fixed resistance R and a variable resistance R",the latter being adjusted to such a value that the current led throughthe resistor R to the adding point will be exactly twice as large atresonance as the current which is led through the inverting transistor Tto the adding point. It is of course alternatively possible to renderthe emitter resistance of the transistor T adjustable. As has beenstated, the relevant amplitude error is independent of the Q, and thecompensation will apply to all Q-values.

Another amplitude error is caused by losses in the resonant circuit.This amplitude error will be dependent on Q as the voltage across theresonant circuit varies with Q. According to FIG. 12 a compensation ofthis error is achieved by means of an added branch which comprises atransistor T and a resistor r which branch is connected in parallel withthe resistor R of the resonant circuit. The value of the resistor r, isadjusted in such a manner that a current is led through this resistor tothe adding point which current is equal to the current lost in the lossresistor of the resonant circuit. Since the voltage across the resonantcircuit varies with Q, the compensation current which is applied throughresistor r, will likewise vary with Q and the compensation will becorrect for all Q-values.

As has been stated, the correction circuit described is principallyadapted for use in TV transmitters, but may in principle also be used inTV receivers.

FIG. 13(a) shows in what manner a sawtooth voltage 8,. representing thevideo signal (first signal) in a color TV system and a chrominancesubcarrier f (second signal) superimposed on the video signal aremodulated on a carrier f This carrier f may be of intermediate frequencyor ultrahigh frequency. The modulation limits U, and U, are chosen tothe white level and the black level, respectively.

If the signal shown in FIG. 13(a) is applied through a transmissionchannel in a TV transmitter and the relative phase of the chrominancesubcarrier for different modulation levels, that is to say, amplitudesof the carrier is investigated, this will result, for example, in acurve as shown in FIG. 13(b). The phase deviation Ad of the chrominancesubcarrier of a reference phase is called a differential phase error.The reference phase is in this case chosen to be equal to the phase atthe lowest amplitude of the carrier which corresponds to the whitelevel. It is evident that the phase deviation in the given example isinsignificant up to approximately half the maximum amplitude, but thenincreases considerably.

If the delay 1 is measured as a function of the angular frequency w inthe case illustrated in FIG. 13(b) this may result, for example, in thecurves shown on the left-hand side of FIG. 14. These are denoted by thereference numerals l to 7 and are supposed to be measured atcorresponding amplitude levels which have the same reference numerals inFIG. 13(b).

According to the definition wherein D is the overall phase shift of thetransmission line. If only the phase deviation relative to the referencephase, that is to'say, the phase at white level amplitude is taken intoconsideration, equation (VI) can be written after integration as whereinAd is the differential phase error and Ar is the deviation in delayrelative to the delay at white level.

According to equation (VII) the differential phase error will berepresented in FIG. 14 for each amplitude in the videosignal by thesurface which is enclosed between the actual delay curve and thehorizontal line 1-,, in the frequency diagram. The phase error MD, forthe highest amplitude of the video signal, that is to say, the blacklevel will thus be proportional to the surface shown in broken lines inFIG. 14(b). This also applies to other amplitude levels. According tothe invention the difierential phase error can be corrected byintroducing a correction circuit in the line which circuit produces anaddition to the delay. A correct correction requires the integral of thedelay curve to be equal to the said integral of the delay curve for thetransmission line, which integral is equal to the differential phaseerror of the transmission line. According to the above the differentialphase error varies, however, with the amplitude of the video signal andthe correction must therefore vary with the amplitude in the samemanner. Mathematically, this can be expressed such that the equation issatisfied for all values of U, wherein U is the amplitude of the carrierand A4 and AD are the said integrals for the transmission line and thecorrection circuit respectively, within the actual frequency band. Ifonly the difierential phase error is taken into account the conditionaccording to equation (VIII) is sufficient. According to the inventionthe differential phase error muy,'however, be corrected by maintaining aconstant delay over the actual frequency band and a constant amplitudeof the output signal for a given input signal.

The correction is performed by means of an active all-pass circuit whichis controllable within certain limits and which may be of a suitablekind as described in FIGS. l to 112. An allpass circuit is thenunderstood to be a circuit which causes a certain addition to the delaywithin a certain frequency range,

but has the same amplification for all frequencies.

The principal circuit for the correction of the differential phase erroraccording to the invention is shown in FIG. I5 The correction issupposed to relate to the phase error currection in a TV transmitter,and at least one correction device according to FIG. I5 is thenconnected to the IF. section of the TV transmitter. The input signal U,is formed by the I.F. carrier together with the video signal and with asuperimposed chrominance subcarrier, which signal is applied on the onehand to an active delay correction circuit Kr according to FIG. 115,which correction circuit will be described in greater detail hereinafterand on the other hand to a demodulator D. The demodulator D supplies anoutput voltage which varies in its rhythm with the video signal andwhich is applied to a device A which produces a suitable control voltageU,. from the video signal for the correction circuit Kr. The correctedsignal U, is derived from the correction circuit Kr.

The delay curve for the correction circuit can be seen in FIG. 1d, whereit is designated 17 The delay curve 1 is varied dependently on the videosignal such that the portion of the curve falling within the actualfrequency range (Dy-( for each value of the amplitude of the videosignal is equal to the phase error at the relevant amplitude. In therelevant case the control of the correction circuit is assumed to beperformed by a displacement in one or the other direction of theresonant frequency m, of the correction circuit. At an increasingamplitude of the video signal, 10,, decreases and at a decreasingamplitude, 10,, increases. The shape of the correction curve 1 isfurthermore assumed to be such that for each adjustment of the circuitit is complementary to the 1 curve transmission line. Hence theresultant delay across the entire frequency range w,-w will be constantfor all amplitudes of the video signal.

FIG. 16 shows a suitable embodiment of the delay correction circuit Kr.The correction circuit comprises a first transistor T which serves as animpedance converter of current generator and a second generator T whichserves as a phase inverter stage. The base of the first transistor T isconnected to earth while the emitter is connected through a resistor R/2to an input terminal to which the combined input signal U, is applied. Aparallel resonant circuit C,, C L, R is incorporated in the collectorcircuit of the transistor T The references C and C are two variablecapacity diodes which are controlled by the common control voltage U Ris connected to the input of the voltage amplifier F having a high gainfactor, the amplifier F is fed back through a resistor R. A very smallsignal voltage occurs at the input of the amplifier F, such that it canbe considered to be earthed, R being effectively connected in parallelwith the parallel arrangement of the capacitors C,, and C and theinductor L.

The base of the second transistor T is directly connected to the inputterminal to which the combined signal U is applied while the emitter isconnected to earth through a resistor R and a decoupling capacitor C Thecollector T is directly connected to the input of the feedback amplifierF. The references R R and R are resistors by which the operating pointsof the two transistors are adjusted and the references C, C and C arecoupling capacitors.

The operation ofthe circuit is as follows:

The transistor T, produces a current I through the resonant circuitwhich current is equal to wherein R,., is the emitter resistance oftransistor T,. A part I, of this current is applied through R to theinput of the amplifier F, I, being determined by the relation llllllwherein C=C +6}.

The transistor T produces a current I to the input of the amplifier I zll. Lil. R r: R

whcrcin r,., is the emitter resistance oftrnnsistor T The sum of thecurrents l,-i-I flows through the feedback resistor R of the amplifierand causes an output voltage which can be written as It is evident fromequation (1X) that the amplification U /U is always equal to for allgrequencies, whereas the phase varies from +1rt0-1r when 5 varies from 0to The function of the circuit is illustrated in a phasor diagram inFIG. 17. With respect to size and phase, 1 represents the voltage acrossthe parallel resonant circuit to which a current I ZUM/R is applied.However, according to equation (VII) the current I will be independentof the value of R When 0) varies from 0 to the phasor I varies in sizeand orientation such that it describes a circle according to FIG. I7.When u is equal to 0, the phase of I, is closed to +rr/2 and when w=close to -1r/2, the phase of the current I being used as a referencephase. At resonance that is to say, the entire current I, flows throughRq to the fed back amplifier.

According to the foregoing the current which is combined with thecurrent from the resonant circuit is equal to half the maximum value ofthe said first current and may be shown as is indicated in the phasordiagram of FIG. 17. It is evident that the phasor Ifl'lg, whichrepresents the output voltage of the circuit, will start from the centerof the said circle, that is to say, it represents a voltage having aconstant amplitude but a varying phase. The phase of the output voltagevaries from +11 at w=0 to 'n' at (ti-= and the delay varies according toa curve which in principle corresponds to the curve which is shown onthe right-hand side of FIG. M.

It can be proved that Q max 2 The magnitude of the addition to the delayproduced by the correction circuit is adjusted by a variation of R andthe position of the said addition along the frequency scale is adjustedby a variation of the resonant frequency. In the relevant case R isconsidered to be adjusted at a suitable value during a previousadjustment, while the resonant frequency is varied by exerting influenceon the variable capacity diodes C,, C in the rhythm of the appliedcontrol voltage U The resonant circuit is formed as a balancedcenter-earthed bridge, the control voltage being applied between thejunction of the two equal capacitors C C and the center of the coil L,which coil is connected to earth with respect to signal voltages. As aresult nothing of the voltage U will appear across the resonant circuitand the voltage U does not contribute to the current I; which is appliedto the amplifier F.

FIG. 18 shows in what manner the differential phase error for atransmission line having a passband characteristic can be corrected withthe aid of two delay correction circuits of the kind described. Thedelay curve for the uncorrected transmission line is shown in the lowerpart of FIG. 18 where it is indicated by 1 The correction circuits areindicated by K1, and K1, and the delay curves for the circuits areindicated by 1 1 and r respectively. The tuning capacitors C C and C,,C, of the correction circuits are adjusted by the common control voltageU the control being such that at a given variation of U, the tuningfrequency of 'one of the circuits will increase and that of the otherwill decrease, that is to say, the tuning frequencies will approach eachother or will be more remote from each other dependent on U,.

The phase error may vary in different manners, as a function of theamplitude and the correction must be adapted in each separate case tothe phase error curve of the associated transmission circuit. FIG. 19shows in what manner a phase error which changes sign, can be corrected.The phase error of the uncorrected transmission line is assumed to varyaccording to the curve indicated by a broken line shown on the lefthandside of FIG. 19. if it is assumed that the relation between the phasecorrection caused by the correction circuit and the applied controlvoltage is linear, a control voltage U will be produced which follows acurve having a function which is opposite to the phase error curve (thecurve indicated by a solid line in FIG. 19).

F l6. 19 shows on the right-hand side in what manner such a controlvoltage can be produced with the aid of two demodulators. Thedemodulators to which the common intermediate frequency signal isapplied are indicated by D, and D respectively. The demodulatorscomprise rectifier elements which are connected with opposite polarity,so that the first demodulator D supplies a positive voltage and thesecond demodulator D supplies a negative voltage. The voltages of thedemodulators are applied through potentiometers P P to an adding devicewhich consists of input resistors R R and an amplifier F which is fedback through a resistor R. The demodulator D produces an output voltagewhich increases linearly with the input amplitude from the value whilethe demodulator D has a threshold value which must be exceeded prior tothe demodulator supplying the output voltage. After the threshold valuehas been exceeded the demodulator D applies a voltage to the addingdevice which voltage increases twice as fast as the voltage of D,. It isevident that the output voltage U,, which is equal to the sum of theinput voltages, will then have a shape as is shown on the left-hand sideof FIG. 19. Any required nonlinear control voltage function can inprinciple be produced with the aid of a number of demodulators whichhave different threshold values and an adding device for adding theoutput voltages of the demodulators so that the correction can beadapted to any measured phase error curve.

Instead of varying the resonant frequency of the correction circuit, itis alternatively possible to vary the Q-value. This is achieved bycontrolling the resistor R of FIG. 16 with the aid of a control signalderived from the video signal. The resistor R may then, for example, beformed as a field-effect transistor.

Both when the resonant frequency of the correction circuit 7 and whenthe Q-value is varied, a large portion of the addition to the delay willlie beyond the actual band limits as shown in the given example. This isdependent on the fact whether the total surface below the delay curvefor the correction circuit is constant, and whether the main part of thedelay curve of the correction circuit must fall within the band limitsof the transmission line, so that only small insignificant variations incould be produced.

The invention is not only limited to the delay correction circuit shown,but also any suitable active controllable correction circuit may inprinciple be used. A requirement is, however, that both the Q-value andthe resonant frequency can easily be adjusted and that at least one ofthe said magnitudes can be adjusted electronically at a speed whichallows of control in the rhythm of the video signal.

In addition to color TV transmitters, the method of correcting thedifferential phase error according to the invention can also be appliedin TV receivers, for example, in the receiver circuit of a slavetransmitter or in common color TV receivers. It is also possible toutilize the correction method in transmitters or receivers formonochrome pictures, the correction in this case and in the exampledescribed being performed at the intermediate frequency level.

What we claim is:

1. A delay distortion correction circuit comprising an input means forreceiving an input signal to be corrected; a current source circuitcoupled to said input means for supplying an output current depending onsaid input signal; a parallel resonance circuit coupled to receive saidoutput current; a damping resistance coupled to said resonant circuit; asecond resistance coupled to said resonant circuit; a currentcombination circuit serially coupled to said damping resistance and alsocoupled to said second resistance; whereby an output signal is obtainedfrom said current combination circuit having a frequency dependent phaserelationship and a frequency independent amplitude relationship withrespect to said input signal.

2. A circuit as claimed in claim 1, wherein said second resistancecomprises the input resistance of said current combination circuit.

3. A circuit as claimed in claim 2 wherein said combination circuitcomprises an amplifier having an input and an output, and a negativefeedback resistor coupled between said input and said output.

4. A circuit as claimed in claim 1 wherein said current source circuitcomprises two transistors coupled in cascode.

5. A circuit as claimed in claim 1 wherein said current source circuitcomprises a transistor and an adjustable resistor coupled between saidtransistor and said input means.

phase 6. A circuit as claimed in claim 1 wherein said parallel resonancecircuit comprises a coil having a tap coupled to ground, one end of saidcoil being coupled through said damping resistance to said currentcombination circuit; and further comprising a variable capacitor coupledbetween said combination circuit and the remaining end of said coil.

7. A circuit as claimed in claim 1 wherein said current source circuitcomprises two current sources.

8. A circuit as claimed in claim 7 wherein at least one of the currentsources is adjustable.

9. A circuit as claimed in claim 1 further comprising a current losscompensation circuit coupled between said resonant and combinationcircuits.

10. A circuit as claimed in claim 1 wherein said resonance circuitcomprises a voltage dependent element.

11. A circuit as claimed in claim 10 further comprising a detectiondevice coupled between said input means and said element.

12. A circuit as claimed in claim 1 further comprising a secondresonance circuit tuned to a different frequency than that of said firstresonant circuit, each of said resonant circuits having a voltagedependent element; and a detection device coupled between said inputmeans and said elements.

13. A circuit as claimed in claim 11 wherein said detection devicecomprises two detection circuits of opposite polarity.

14. A circuit as claimed in claim 11 wherein said detection devicecomprises a plurality of detection circuits each having a differentthreshold, and an adding circuit coupled between said detection circuitsand said elements.

III '0' l

1. A delay distortion correction circuit comprising an input means forreceiving an input signal to be corrected; a current source circuitcoupled to said input means for supplying an output current depending onsaid input signal; a parallel resonance circuit coupled to receive saidoutput current; a damping resistance coupled to said resonant circuit; asecond resistance coupled to said resonant circuit; a currentcombination circuit serially coupled to said damping resistance and alsocoupled to said second resistance; whereby an output signal is obtainedfrom said current combination circuit having a frequency dependent phaserelationship and a frequency independent amplitude relationship withrespect to said input signal.
 2. A circuit as claimed in claim 1,wherein said second resistance comprises the input resistance of saidcurrent combination circuit.
 3. A circuit as claimed in claim 2 whereinsaid combination circuit comprises an amplifier having an input and anoutput, and a negative feedback resistor coupled between said input andsaid output.
 4. A circuit as claimed in claim 1 wherein said currentsource circuit comprises two transistors coupled in cascode.
 5. Acircuit as claimed in claim 1 wherein said current source circuitcomprises a transistor and an adjustable resistor coupled between saidtransistor and said input means.
 6. A circuit as claimed in claim 1wherein said parallel resonance circuit comprises a coil having a tapcoupled to ground, one end of said coil being coupled through saiddamping resistance to said current combination circuit; and furthercomprising a variable capacitor coupled between said combination circuitand the remaining end of said coil.
 7. A circuit as claimed in claim 1wherein said current source circuit comprises two current sources.
 8. Acircuit as claimed in claim 7 wherein at least one of the currentsources is adjustable.
 9. A circuit as claimed in claim 1 furthercomprising a current loss compensation circuit coupled between saidresonant and combination circuits.
 10. A circuit as claimed in claim 1wherein said resonance circuit comprises a voltage dependent element.11. A circuit as claimed in claim 10 further comprising a detectiondevice coupled between said input means and said element.
 12. A circuitas claimed in claim 1 further comprising a second resonance circuittuned to a different frequency than that of said first resonant circuit,each of said resonant circuits having a voltage dependent element; and adetection device coupled between said input means and said elements. 13.A circuit as claimed in claim 11 wherein said detection device comprisestwo detection cirCuits of opposite polarity.
 14. A circuit as claimed inclaim 11 wherein said detection device comprises a plurality ofdetection circuits each having a different threshold, and an addingcircuit coupled between said detection circuits and said elements.